This invention relates generally to controlling fuel flow to combustors in gas turbine engines that have pilot fuel flow and premix fuel flow and in particular to a control for adjusting the split between the premix and pilot fuel flows to control flame temperature and thus minimize emissions.
Air pollution concerns worldwide have led to stricter emissions standards requiring significant reductions in gas turbine pollutant emissions for both industrial and power generation applications burning either liquid or gaseous fuel.
Among other factors, combustion emissions are dependent upon combustor flame temperature and in particular the peak flame temperature in the primary combustion zone of the combustor. Higher flame temperatures result in lower carbon monoxide (CO) emissions but higher nitrous oxide (NOx) emissions. Depending on the type of fuel used, there is a desired flame temperature wherein the combination of carbon monoxide and nitrous oxide is minimized. This relationship is illustrated in FIG. 1 in which curves of CO and NOx emissions, in parts per million, are plotted versus flame temperature. From this curve an ideal flame temperature can be selected, represented by the dashed line, that minimizes the combination of CO and NOx emissions. Alternatively, the flame temperature can be selected either to the left or right of dashed line for improved CO or NOx emissions at the expense of the other.
One proposed solution to reduce emissions is the use of combustors that operate with two different fuel flows. One of these fuel flows is referred to as pilot fuel flow. This fuel flow is injected directly into the combustion chamber and is the only fuel flow during the starting of the engine. Once the engine has reached a certain percent of its operating speed, premix fuel flow is added to the combustion chamber in addition to the pilot fuel. This premix fuel flow is mixed with air prior to entering the combustion chamber, hence the name xe2x80x9cpremixxe2x80x9d. By controlling the ratio of pilot flow to premix flow, the primary zone peak temperature can be adjusted, and therefore, the level of CO and NOx emissions. An example of this type of combustor is disclosed in copending patent application Ser. No. 09/493,646, filed on Jan. 28, 2000, entitled xe2x80x9cLow Cost, Low Emissions Natural Gas Combustorxe2x80x9d and which is assigned to the Assignee of this application.
Accordingly, there is a need for a control system and method for controlling the split between the premix fuel flow and the pilot fuel flow to keep emissions down.
An object of the present invention is to provide a control system and method for controlling the split between the premix fuel flow and the pilot fuel flow to keep emissions down. As used herein, the term xe2x80x9cpremix splitxe2x80x9d is the ratio of premix fuel flow to total fuel flow; xe2x80x9cpilot splitxe2x80x9d is the ratio of pilot fuel flow to total fuel flow; and xe2x80x9csplitxe2x80x9d is used to indicate either premix split or pilot split, as appropriate in the context.
The present invention achieves this objective by providing a control system that controls the split between premix fuel flow and pilot fuel flow to a combustor in a gas turbine engine so that an acceptable level of emissions are maintained. This is accomplished by first sensing an operating condition of the engine such as by using engine speed and exhaust gas temperature. Using these sensed parameters, the combustor pilot zone temperature is calculated. In the preferred embodiment, the pilot zone temperature is obtained from the following equation:
Tpz=(xcex8EGT)k1xc3x97(N/xcex80.5÷K2).
From this pilot zone temperature a desired split between premix fuel flow and pilot fuel flow is obtained that will result in the desired level of emissions, and compared to the actual split to generate an error signal. The actual premix fuel flow and pilot fuel flow are then adjusted until the desired split and hence the desired emission levels are obtained.
These and other objects, features and advantages of the present invention, are specifically set forth in, or will become apparent from, the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings.